244-248] Light produced by Heating 207 



PHYSICAL CONSEQUENCES OF HYPOTHESES. 



246. We shall conclude the present chapter with the mention of a few 

 physical considerations which arise from this hypothetical view of the -J/ 

 structure of the molecule. They have very little bearing on the direct 

 development of the Kinetic Theory, except in so far as they tend to 

 strengthen the evidence for the hypotheses on which they rest. 



Calculation of Temperature of Incandescence. 



247. In 242, we assumed the value 0=5 x 10 4 corresponding roughly 

 to air under normal conditions. If, instead of this we had taken (7=5 x 10 5 

 corresponding to air at a temperature of 28,000 C., the value obtained for 

 formula (498) in expression (499) would have to be replaced by 



13e~ 12 , 



of which the value is about '00008, a quantity which is certainly not negli- 

 gible for our present purpose. For if we take the number of collisions 

 per second to be 10 10 we find that the translational energy is reduced to 

 e" 1 times its initial value in about a millionth of a second. We should 

 therefore expect a gas at a temperature comparable with 28,000 to lose 

 energy with extreme rapidity. Our hypothesis would then lead to the 

 prediction of a temperature of incandescence a temperature, that is, at 

 which the loss of energy by radiation becomes perceptible lying some- 

 where between C. and 28,000 C. Stated in this form, the result may 

 seem at first sight to be almost worthless as a confirmation of the hypothesis 

 on which it rests. It ought, however, to be noticed that the hypothesis 

 enables us to predict the value of G corresponding to the temperature of 

 incandescence to within less than a power of 10 a result sufficiently 

 remarkable when we consider the vagueness of the numerical data on which 

 it rests. 



Experiments of Schuster and Hemsalech. 



248. The collisions between molecules moving with low velocities will 

 give rise to but little radiation for reasons already explained : those between 

 molecules moving with high velocities will be extremely rare. Hence the 

 greater amount of the radiation will, when the radiation is perceptible, arise 

 from the collisions of molecules moving with certain intermediate velocities ; 

 and, on account of the rapidity of change of the exponential expressions 

 (cf. expressions (492) and (498)) on which the radiation depends, it is clear 

 that the velocities will be closely grouped about a certain mean velocity. 

 This mean velocity will, as is clear from an inspection of expression (492), 



